Not Applicable.
Not Applicable.
1. Field of the Invention
This invention relates to concrete and controlled low-strength materials having increased electrical conductivity such that when used in construction, the material is capable of conducting electrical charges such as those resulting from a lightning strike. Further, the concrete and controlled low-strength materials include a high carbon content fly ash, thus providing a means for utilization of a product usually considered a by-product, or a waste product, of coal burning power generation.
2. Description of the Related Art
It is known that fly ash can be incorporated into concrete. See, for example, U.S. Pat. Nos. 6,461,424, 4,116,705, 4,268,316, 5,520,730, 5,853,475, 5,346,012, 5,490,889, 5,374,308, 4,230,568, 4,050,261 and 4,210,457; European patent application EP 744386; Davis et al., xe2x80x9cWeathering Resistance of Concretes Containing Fly-Ash Cementsxe2x80x9d, Journal of the ACI, vol. 12, pages 281-293, 1941; Timms et al., xe2x80x9cUse of Fly Ash in Concretexe2x80x9d, ASTM Proceedings, 1956; and Cabrera et al., xe2x80x9cDesign and Properties of High-Volume Fly Ash High-Performance Concretexe2x80x9d, American Concrete Institute, SP 186-2, p. 21-37, 1999. In most of these patents and publications, the fly ash utilized comprises any of those fly ashes which meet the requirements of ASTM (American Society for Testing and Materials) C 618, xe2x80x9cStandard Specification for Fly Ash and Raw or Calcined Natural Pozzolan for Use as a Mineral Admixture in Portland Cement Concrete.xe2x80x9d
It is also known that fly ash can be incorporated into controlled low-strength materials (often referred to as xe2x80x9cCLSMxe2x80x9d). In the publication xe2x80x9cControlled Low-Strength Materialsxe2x80x9d, reported by American Concrete Institute Committee 229, June 1999, there is provided a description of controlled low-strength materials along with certain ingredient mixtures used to produce CLSM. Controlled low-strength materials are broadly defined in this publication as self-compacted, cementitious materials used primarily as a backfill in place of compacted fill. Conventional CLSM mixtures usually consist of water, portland cement, fly ash, and fine or coarse aggregates. Some CLSM mixtures consist of water, portland cement and fly ash. However, CLSM is not to be considered as a type of low-strength concrete. This publication also defines CLSM as a material that results in a compressive strength of 8.3 MPa (1200 psi) or less at the conventional 28 day testing period (typically without compaction), and notes that most current CLSM applications require unconfined compressive strengths of 2.1 MPa (300 psi) or less at the conventional 28 day testing period in order to allow future excavation. This publication makes reference to certain examples of CLSM mixtures which include fly ash. U.S. Pat. Nos. 5,951,751 and 4,374,672 also disclose the use of fly ashes which meet the requirements of ASTM C 618 in controlled low-strength materials.
It is also known that fly ash is a voluminous by-product of coal burning electrical power generation plants, presenting a possible environmental disposal issue. While those fly ash varieties which meet the requirements of the ASTM Standard Specification C 618 for classes C and F are used as additives to concrete, those fly ash materials which have an excessively high carbon content may not be so used. Accordingly, much of this type of fly ash is relegated to land fill, a less than desirable solution from an environmental viewpoint.
Air dried concrete is considered a reasonably good electrical insulator, having a resistivity on the order of 106 ohm-cm, with oven dried concrete having a resistivity on the order of 1011 ohm-cm. Moist concrete, on the other hand is an electrolyte having a resistivity on the order of 104 ohm-cm, which leads to its classification as a semiconductor. Since the transmission of electrical charge in moist concrete occurs through the movement of dissolved ions in the electrolytic solution, higher cement content and higher water content result in lower resistivity. High water content, however, is not acceptable for structural concrete, since it also results in lowered compressive strength and density. It has been found that there is a direct relationship between the degree of hydration of the cement paste and resistivity, yielding a linear relationship between resistivity and compressive strength of cement paste and concrete. That is, resistivity increases as the compressive strength increases.
Electrically conductive concrete may be produced by placing electrically conductive fibers and/or particles in close contact with each other so that a conductive network may be formed throughout the concrete. In conductive concrete, the transmission of electrical charge occurs mainly through the conductive additives, rather than through the electrolytic solution created in moist concrete. Such additives as carbon fibers, steel fibers, steel shavings, and carbon black have been found to be effective in modifying the conductivity of concrete into which they are blended. For example, U.S. Pat. No. 3,962,142 teaches the use of calcined oil coke and acetylene black aggregates in conductive concrete having satisfactory mechanical strength, while U.S. Pat. No. 5,908,584 teaches a mixture of graphite, amorphous carbon, and sand, comprising 25 to 75% of a cementitious composite useful for conducting floors, heating elements, and ground connectors.
Electrically conductive concrete and controlled low-strength materials would be advantageous where lowered electrical resistance may be sought, such as for use in structures where it is necessary to protect electrical equipment from lightning strikes. Accordingly, a means to reduce the electrical resistance of concrete or controlled low-strength materials, or to increase the conductivity thereof, is of interest in the building industry, for example. Further, since high carbon content fly ash is readily available as a waste product, and carbon is known to be highly conductive, the use of fly ash and carbon fibers as additives to concrete or controlled low-strength materials to lower electrical resistance have now been investigated.
The need for concrete having increased electrical conductivity is met by a composition according to the invention that is capable of setting to produce electrically conductive concrete. The composition includes from about 1% to about 30% by weight of portland cement; from about 1% to about 30% by weight of fly ash having a carbon content as measured by loss on ignition of greater than 12%; from about 40% to about 90% by weight of an aggregate; from about 0.1% to about 20% by weight of carbon fibers; and water in a sufficient amount such that the composition sets to a concrete having a compressive strength of at least 13.8 MPa, wherein all weight percentages are percent by weight of the total composition.
In another aspect, the invention satisfies the need for a controlled low-strength material having increased electrical conductivity. In this regard, the invention provides a self-compacting, cementitious flowable fill composition that includes from about 1% to about 30% by weight of portland cement; from about 5% to about 85% by weight of fly ash; from about 0.1% to about 20% by weight of carbon fibers; and water in a sufficient amount such that the composition sets to a material having a compressive strength of 8.3 MPa or less, wherein all weight percentages are percent by weight of the total composition. Preferably, the fly ash has a carbon content as measured by loss on ignition of greater than 12%.
Thus, it is an advantage of the present invention to provide an inexpensive method for providing electrically conductive construction materials.
It is a further advantage to provide an economically advantageous means of beneficial utilization of a waste by-product.
It is another advantage to provide electrically conductive concrete that can be inexpensively manufactured, and to provide a dry concrete mixture that may be packaged for sale to the public for individual use by consumers wishing to provide an electrically conductive concrete structural element.
It is yet another advantage to provide a dry mix for a conductive concrete which can be inexpensively packed, delivered, mixed and used.
These and other features, aspects, and advantages of the present invention will become better understood upon consideration of the following detailed description and appended claims.